1. FIELD OF THE INVENTION
This invention relates to a process for producing powders of ferromagnetic metals, and more particularly to a process for producing powders of ferromagnetic metals having a square hysteresis loop suited for use in magnetic recording tape, a high coercive force and high maximum residual flux density.
2. DESCRIPTION OF THE PRIOR ART
Feroxide ferromagnetic powders, such as .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 hitherto used for producing magnetic recording tape, are not suited for recording signals of ultra-short wave lengths (less than 10 .mu.) or high density magnetic recording since the coercive force and the maximum residual flux density thereof are insufficient.
Recently, many studies have been made to find ferromagnetic materials having magnetic properties suited for high density magnetic recording. Suitable ferromagnetic metal powders discovered include pure metal powders such as Co powder or Ni powder, and alloy powders composed mainly of at least two metals selected from Co, Ni and Fe.
These ferromagnetic powders are produced by the following known processes:
1. Reducing the oxalate of a metal capable of forming the ferromagnetic body in flowing H.sub.2 gas at high temperatures. (see Japanese Patent Publications 11412/61, 22230/61, 8027/65, 14818/66 and 22394/68)
2. Reducing goethite or accicular .gamma.-Fe.sub.2 O.sub.3 in flowing H.sub.2 gas at high temperatures. (see Japanese Patent Publications 3862/60 and 20939/64)
3. Evaporating a ferromagnetic metal in an inert gas atmophere. (see "Applied Physics" vol. 40, No. 1, p, 110 (1971))
4. Reducing a salt of a metal capable of forming the ferromagnetic body in a solution of the salt using a borohydride. (see Japanese Patent Publications 20116/68 and 26555/63, and "Television", Vol. 19, No. 1, p, 19 (1965))
5. Decomposing a carbonyl of a metal capable of forming the ferromagnetic body. (see U.S. Pats. Nos. 2,983,997, 3,172,776, 3,200,007, and 3,228,882)
6. Electrolytically depositing a ferromagnetic metal using an Hg cathode and separating the deposited metal from Hg by heating. (see Japanese Patent Publications 15525/64 and 8123/65)
According to processes (1) and (2) the volume of the metal powder is decreased during the high temperature reducing treatment, thus causing intergranular voids or aperatures, sintering of the powder, activation of the powder surface and deformation of the powder shape are caused during the reducing treatment at temperatures higher than 300.degree. C in the flowing H.sub.2 gas, thus causing irregular dispersion of the ferromagnetic powder in a binder and causing the ferromagnetic powder to exhibit insufficient ferromagnetic properties.
According to processes (1), (2) and (3) the metal powder after the reducing or evaporating treatment poses a danger of ignition due to the highly active powder surface, and accordingly treating and handling of the metal powder are very disadvantageousfrom the commercial viewpoint.
Although wet processes (4), (5) and (6) eliminate the inherent defects of dry processes (1), (2) and (3), the ferromagnetic metal powder obtained, for example by process (4), is accicular and easily broken during mixing and dispersing in a binder, thus lowering the orientation property in a magnetic field. This is observed as an inferior squareness ratio (Br/Bs).
In addition, processes (5) and (6) require careful handling or poisonous and dangerous materials such as metal carbonyls and mercury.